The present invention relates generally to a method for reducing the carbon content of molten steel by the employment of vacuum degassing and more particularly to such a method in which oxygen blowing is also employed. A method of this type is conventionally known as an RH--OB process.
The RH--OB process employs a vertically disposed treatment vessel from the bottom of which extend a pair of tubular members having open lower ends and which function as siphon tubes or snorkels. The treatment vessel is disposed directly above a ladle which contains a bath of molten steel covered with slag and including carbon and dissolved oxygen. The ladle is raised until the lower open ends of the two snorkels extend below the surface of the molten steel. The interior of the treatment vessel is evacuated through an exhaust outlet located near the top of the vessel. The atmospheric pressure on the molten steel in the ladle causes the molten steel and slag cover to rise upwardly through the two snorkels into the interior of the treatment vessel which has been evacuated to a sub-atmospheric pressure. An inert gas, such as argon or nitrogen, is introduced into one snorkel to reduce the density of the molten steel therein, and molten steel in that snorkel rises relative to the molten steel in the other snorkel. The net result is that less dense molten steel enters the treatment vessel through the one snorkel (the inlet snorkel) and denser molten steel exits the treatment vessel through the other snorkel (the exit snorkel).
In this manner molten steel is circulated from the ladle upwardly through the inlet snorkel into the interior of the treatment vessel and then downwardly through the exit snorkel back into the ladle. When the molten steel is in the treatment vessel it undergoes a process which reduces the carbon and dissolved oxygen contents of the molten steel, in a manner to be described below. The introduction of inert gas into the molten steel at the inlet snorkel increases the surface area of steel exposed to the reduced pressure within the treatment vessel, thereby facilitating the treatment.
Continuing the circulation procedure for a period of time results in the entire volume of the molten steel in the ladle being subjected to the treatment. Typically, the total contents of the ladle, usually an entire heat of steel from a basic oxygen furnace (e.g. 200 tonnes of molten steel), is circulated through the treatment vessel in a few minutes or less. The bath of molten steel in the ladle is typically recirculated through the treatment vessel number of times for as long as it takes to reduce the carbon content to the desired level, and the treatment time can be 10-30 minutes, for example.
The molten steel subjected to the treatment described above is non-deoxidized before the treatment begins, i.e. the molten steel contains a substantial quantity of dissolved oxygen which has not been removed by reaction with a solid deoxidizing agent such as aluminum or silicon. The carbon content in molten steel containing dissolved oxygen is reduced when the molten steel is subjected to a vacuum degassing operation. More particularly, the dissolved oxygen in the molten steel reacts with the carbon to form carbon monoxide which leaves the molten steel in the treatment vessel and is exhausted therefrom. This is known as natural decarburization. Steel with very low carbon contents (i.e. 0.002 wt.%) can be produced under these conditions.
The reaction between carbon and oxygen to form carbon monoxide (CO) is an equilibrium reaction which can move in either direction (C+O.revreaction.CO). The direction in which the reaction occurs is related to the partial pressure of carbon monoxide in the atmosphere above the molten steel in the treatment vessel. Initially, the molten steel is saturated with dissolved oxygen so that lowering the partial pressure of carbon monoxide by exhausting CO from the vessel drives the reaction to produce carbon monoxide. Because the carbon monoxide is continuously withdrawn, thereby maintaining a relatively low partial pressure of CO within the treatment vessel (e.g. 200 Torr. or less), the reaction is continuously driven in a direction to form carbon monoxide, and this removes both dissolved oxygen and carbon from the molten steel. The net result is to reduce substantially both the carbon content and the dissolved oxygen content of the molten steel.
Generally, the lower the partial pressure of CO, the lower the carbon content when equilibrium is attained.
The process described above is known as the RH process. A refinement of this process is known as the RH--OB process in which the treatment vessel is equipped with oxygen tuyeres or blowers in the sides of the vessel, at the lower part thereof. Oxygen can be blown through these tuyeres into the molten steel in the treatment vessel, and this provides several potential benefits.
More particularly, the oxygen can be utilized to accelerate decarburization, and this is known as forced decarburization. Forced decarburization provides faster processing of the steel in the vacuum degassing vessel, which is desirable in that it maximizes utilization of downstream casting equipment, such as a continuous caster which can be scheduled to continuously cast, without interruption, the molten steel from the degassing vessel. In addition, the untreated molten steel can be tapped from the basic oxygen furnace at a significantly higher carbon level and a significantly lower dissolved oxygen level, than when the molten steel from the basic oxygen furnace is to be subjected to a vacuum degassing treatment not employing oxygen blowing, and that is desirable. Oxygen blowing increases the amount of carbon which can be removed from the molten steel by vacuum degassing at a given sub-atmospheric pressure. Oxygen blowing also reduces the time period required to reduce carbon to the desired level, at a given partial pressure.
Moreover, in a treatment vessel equipped to provide oxygen blowing, the molten steel undergoing treatment can be reheated employing a process called aluminum reheating in which aluminum is added to the molten steel and oxygen is blown through the tuyeres causing a reaction, between the aluminum and the oxygen, which is exothermic and produces heat. Aluminum reheating is disadvantageous in some respects because, aluminum being relatively expensive, its employment as a heat source is an expensive way to obtain energy. In addition, the aluminum oxide formed by the reaction during aluminum reheating must be flushed from the molten steel into the slag cover on the molten steel, and this requires additional recirculation time which in turn prolongs the process. Oxygen blowing itself has a drawback in that it has an adverse affect on refractory life in the treatment vessel.
The temperature at which the treatment is conducted depends upon the temperature at which the steel is to be cast following treatment. The casting temperature is usually about 60.degree. C. higher than the solidus temperature of the steel. It is desirable to start the treatment at a temperature no lower than the casting temperature.
In conventional practice, if the molten steel arrives at the treatment vessel with a temperature which is too low or with a carbon content which is outside the range of initial carbon content for which the treatment has been designed, a pretreatment is performed to bring the starting temperature and the starting carbon content into the ranges desired. The carbon content can be reduced or increased during the pre-treatment by oxygen blowing or coke addition respectively. The temperature can be raised by aluminum reheating.
Alloy additions to the molten steel are made in the treatment vessel, after the pressure has been reduced and recirculation is occurring.
Following the decarburization part of the treatment, the remaining dissolved oxygen content is eliminated by adding a solid deoxidizing agent such as aluminum. This, of course, forms aluminum oxide, and substantial amounts of aluminum oxide inclusions in the finished steel product are undesirable. Prolonged additional recirculation of the molten steel to flush the aluminum oxide into the slag is undesirable because it delays casting resulting in down time for the casting equipment.
At the conclusion of the treatment, after deoxidation with aluminum to remove the residual dissolved oxygen, and after additional recirculation to flush the aluminum oxide inclusions into the slag, the vessel is repressurized, and all the molten steel descends into the ladle. The ladle is then moved to a casting station at which the molten steel is withdrawn from the ladle into either ingot molds or into a continuous caster.
The time period for the treatment is usually set by scheduling considerations at a casting station downstream of the vacuum degasser. Accordingly, an operating vacuum (partial pressure) or a combination of operating vacuum and oxygen blowing (if the latter is available) is selected which will reduce the carbon content to the desired level in the period of time available.
In conventional practice, measurements are made of the temperature and carbon and dissolved oxygen contents of the molten steel leaving the basic oxygen furnace. Calculations are made of the predicted end points for carbon and temperature after vacuum degassing, based upon the processing conditions to be employed and upon certain extraneous factors such as required alloying additions. The calculations for carbon and temperature treat the changes in carbon and temperature as functions of time, and they are made independently of each other. A similar independent calculation is made of the predicted dissolved oxygen end point. If the predicted end points for carbon and temperature don't come within permissible limits of the aim end points for carbon and temperature, processing adjustments are made, either during the pre-treatment or during the treatment itself, to bring the predicted end points closer to the aim end points.
However, when following conventional practice, the actual results obtained at the end of decarburization do not conform sufficiently closely to the aim. There are substantial differences between (a) actual carbon and dissolved oxygen contents and (b) predicted carbon and dissolved oxygen contents, both in average relative error and in standard deviation of the relative error, over a number of heats. In addition, the end temperature cannot be accurately predicted.
In conventional practice, the calculations employed to predict end points do not include a number of important factors such as the effect of changes in pressure on the decarburization rate and on changes in the dissolved oxygen content. The carbon/oxygen equilibria, at the different pressures and temperatures occuring during the treatment, are not taken into account. Although conventional practice includes oxygen return from the slag to the molten steel as a factor in calculating the amount of decarburization, conventional practice does not include the effect of reduced pressure on oxygen return from the slag. Conventional practice does not employ the carbon/oxygen equilibrium curve in predicting carbon and dissolved oxygen end points, or the effect of oxygen-consuming additions (such as a manganese alloy) on the dissolved oxygen content. The calculations for carbon, dissolved oxygen and temperature are not solved simultaneously, and the effect of dissolved oxygen content on the process is largely ignored. Dissolved oxygen is not adjusted or otherwise addressed as a process controlling parameter in conventional practice which also largely ignores the effect of carbon, dissolved oxygen and temperature on each other when controlling or adjusting carbon or temperature.